1. Field of the Disclosure
Embodiments of the present disclosure relate generally to horizontal directional drilling reamers. More particularly, embodiments of the present disclosure relate to methods and apparatus to minimize movement of cutting leg assemblies mounted on directional drilling reamers.
2. Background Art
Horizontal directional drilling (“HDD”) is a process through which a subterranean bore is directionally drilled in a substantially horizontal trajectory from one surface location to another. Typically, HDD operations are used by the utilities industry to create subterranean utility conduits underneath pre-existing structures, but any application requiring a substantially horizontal borehole may utilize HDD. Frequently, HDD bores are drilled to traverse rivers, roadways, buildings, or any other structures where a “cut and cover” methodology is cost prohibitive or otherwise inappropriate.
During a typical HDD operation, a horizontal drilling rig drives a drill bit into the earth at the end of a series of threadably connected pipes called a drillstring. As the operation is substantially horizontal, the drilling rig supplies rotational (torque on bit) and axial (weight on bit) forces to the drill bit through the drillstring. As the drill bit proceeds through the formation, additional lengths of drill pipe are added to increase the length of the drillstring. As the drillstring increases in flexibility over longer lengths, the drillstring can be biased in a predetermined direction to direct the path of the attached drill bit. Thus, the drilling is “directional” in that the path of the bit at the end of the drillstring can be modified to follow a particular trajectory or to avoid subterranean obstacles.
Typically, HDD operations begin with the drilling of a small “pilot” hole from the first surface location using techniques described above. Because of the diminished size in relation to the final desired diameter of the borehole, it is much easier to directionally drill a pilot bore than a full-gage hole. Furthermore, the reduced size of the pilot bit allows for easier changes in trajectory than would be possible using a full-gage bit. At the end of the pilot bore, the drillstring emerges from the second surface location, where the pilot bit is removed and a back reamer assembly is installed. Usually, the back reamer assembly is a stabilized hole opener that is rotated as it is axially pulled back through the pilot bore from the second surface location to the first surface location. The drilling rig that supplied rotary and axial thrusting forces to the pilot bit during the drilling of the pilot bore supplies rotary and axial tensile forces to the back reamer through the drillstring during the back reaming.
Referring now to FIGS. 1A-1C, side views of cutting leg assemblies 12 mounted on a back reamer are shown indicating loads applied on cutting leg assemblies 12 during operation. During HDD operations, stressing and cracking may occur in retention arrangements (e.g., welds) that secure cutting leg assemblies 12 to receptacles 10 of a main reamer body 6. As shown, normal cutting loads “C” are applied on cutting leg assembly 12 due to contact between cutters on the rotating cutter body 16 and the borehole being drilled. Additionally, dead weight of the entire reamer (some reamers may weight up to 12,000 pounds or more) during each revolution and vibrations during operation combine to form a dynamic load “D,” which causes leg movement within the receptacles. Dynamic load D (and resulting stresses) varies from minimum to maximum and again to minimum at least once during one revolution of the reamer as the reamer rotates in the borehole and the cutting leg assembly moves into and out of contact with the borehole.
Dynamic loads D may be typically concentrated in an area where rotating cutter body 16 (cone) attaches to cutter leg 14 because the region where rotating cutter body 16 attaches to cutter leg 14 is closest to the borehole wall (due to protrusion of cutter body 16 in a radial direction). As shown in FIG. 1B, as dynamic loads D are applied, a front edge of the receptacle acts as a fulcrum “F” and a back end of cutting leg assembly 12 is pushed or lifted out of receptacle 10 in a direction generally perpendicular to the reamer axis 1, or radial direction. This movement of cuffing leg assembly 12 inside receptacle 10 causes stressing of retention methods. Cracks are observed in welded reamer at weld locations “W,” as shown in FIG. 1C. Stressing and subsequent cracking of the welds may typically start at the back of the cutting leg assembly 12 (end opposite the cutter body 16) and separation of the cutting leg assembly 12 from the receptacle may be highest in this location.
Accordingly, there exists a need for method and apparatus to mitigate weld cracking between reamer bodies and cutting leg assemblies.